The Type 205 was a class of diesel-electric German hunter-killer U-boat submarines. They were single-hull vessels optimized for the use in the shallow Baltic Sea. The Type 205 is a direct evolution of the Type 201 class with lengthened hull, new machinery and sensors. The biggest difference though is that ST-52 steel is used for the pressure hull since the Type 201's non-magnetic steel proved to be problematic. Type 206, the follow-on class, finally succeeded with non-magnetic steel hulls.The Type 205 was in service with the Royal Danish Navy until 2004, in which it was known as Narhvalen class. The Danish boats differed slightly from the German ones to meet special Danish demands.